This invention is directed to a process for converting crude quinacridone to pigmentary form and, more particularly, to an improved process employing relatively low levels of inorganic salt.
It is well known in the art that quinacridones direct from synthesis, known as crude quinacridones, are unsuitable for many pigmentary uses and must be further processed to develop appropriate pigmentary properties, e.g., particle size, strength, phase, etc. The most commonly used processes for converting curde quinacridone to pigmentary form involve milling the crude quinacridone with large quantities of inorganic salt, e.g., from 150% to 900% based on pigment weight, then acid extracting the resulting quinacridone pigment. While the salt-grinding process can produce satisfactory pigment, the total volume of the grinding elements and salt is so large that only a relatively small amount of pigment can be milled for each batch. Furthermore, disposal of the large amounts of salt solution which results from the extraction step creates potential pollution problems and adds considerably to the manufacturing costs of the pigment.
An early attempt to eliminate the high salt levels traditionally thought to be required in the processing of quinacridone, described in U.S. Pat. No. 2,857,400, involved milling crude quinacridone without salt and contacting the milled quinacridone with an organic liquid to promote development of pigmentary properties. This method was later improved by the discovery that the quinacridone could be contacted with an organic liquid in water emulsion, as described in U.S. Pat. No. 3,017,414.
While the quinacridone pigments prepared by the salt-free processes are satisfactory for a variety of applications where pigment masstone is not particularly critical, in applications requiring particularly dark masstones, such as automotive finishes, the masstone often is insufficiently dark.
This invention provides for an improved process for converting crude quinacridone to pigmentary form whereby the dark masstone characteristic of salt-milling processes is achieved while salt pollution is minimized.